Medroxyprogesterone acetate (MPA) and melengestrol acetate (MGA) were evaluated as contraceptives in cotton-top tamarins (Saguinus oedipus). Progestational drugs are safe and effective contraceptives in man but only limited empirical data are available for tamarins. There were three experimental groups A) bimonthly injection of MPA (Depo-ProveraR) B) MGA implant (Marshfield Medical Foundation, 0.5 gm) and C) no treatment. Serum progesterone concentration was determined in all animals on 4 consecutive 7 day intervals beginning 4 weeks after treatment of Group A. Progesterone concentration > 10 ng/ml indicated a functional corpus luteum. Normal ovarian cyclicity remained in 4/4 tamarins receiving 3 mg/kg MPA and 3/4 receiving 10 mg/kg. MGA suppressed ovarian cyclicity in 5/5 tamarins. Normal ovarian cyclicity remained in 4/5 controls. Ovarian cyclicity returned in a previously noncycling female house with a female that received MGA. Contraceptives were withdrawn after one year of therapy. Bilateral ovariohysterectomy was performed in two members of each experimental group. The uterus and ovaries were examined histologically. One control ovary showed evidence of recent ovulation; the other control animal and all contraceptive recipients had tertiary follicles. All contraceptive recipients and one control animal exhibited proliferative phase endometria. The two MGA recipients also exhibited endometrial glandular ectasia. After contraceptives were withdrawn two MGA recipients, 1 MPA recipient and two control animals went into breeding and became pregnant.